Rad-Hard Dual-Threshold High Count Rate Silicon Pixel Array Detector
Available 2D multi-wire proportional counters (MWPC) are limited by their low global count-rate capability and are insufficient for imaging X-raycrystal spectroscopy (XCS) experiments. Solid state X-ray pixilated array detectors have been shown to overcome this problem. However, large area solid state 2D detectors with higher performance, increased functionality, and programmable, dual-threshold capabilities are needed to achieve the energy windowing necessary for thermonuclear fusion and plasma physics instruments. Because of the harsh environment inside the tokamak vacuum vessel, these detectors must also function and perform with high reliability in conditions not previously encountered by diagnostic technology, including high gamma and neutron environments. Voxtel is developing a large area single photon counting CMOS imager for X-ray imaging spectroscopy. Each pixel of the X-ray photon counter is configured with spectroscopic amplifier features, including low-noise signal amplification with programmable gain/bandwidth preamplifier, and pulse shaping circuits. Enhanced X-rayphoton counting functions include two programmable pulse discrimination circuits (comparators) and two programmable, separately-gateable 15-bit counters. The counters can be operated individually, each connected to the high and low energy comparators, respectively; in this configuration, they can also be gated individually, or the counters can be configured as a single 30-bit. The threshold comparators can be programmed using column-specific global digital-to-analog converters, and each pixel includes additional 3-bits of programmability. Another included feature is `integrate-while-read, which makes the dead time between frames negligible. The readout integrated circuits (ROICs) are designed to be mosaiced, to form a monolithic detector with no dead space between individual ROICS. The imager is specifically designed for operating in the high gamma and neutron environments of a tokamak. In Phase I, Voxtel tested a prototype dual-threshold X-ray imager and its performance, including noise and linearity. We designed and fabricated programmable test camera electronics, the ROIC specifications where shown to meet the application requirements, and we made thick, fully-depleted detectors using high-resistivity Silicon. We identified a number of product enhancements, and designed and simulated the Phase II imager circuits. Commercial Applications and Other Benefits: The innovation has application to a range of high energy physics, nuclear physics, and other science applications. The innovation also has applicability for medical applications, including digital mammography, PET, SPECT, and others. Other applications include homeland security, nuclear weapons monitoring, as well as industrial inspection.
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